In the February 15/22, 2021 issue of The New Yorker there is a photo essay by Brendan George Ko of the annual migration of monarch butterflies, with accompanying text by Carolyn Kormann.
The butterflies have never seen the forest before, but they know—perhaps through an inner compass—that this is where they belong. an inner compass—that this is where they belong. They leave Canada and the northeastern United States in late summer and fly for two months, as far as three thousand miles south and west across the continent. The migration is accomplished in a single generation that lives eight months, whereas the return journey north will occur over some four generations, each living four to five weeks. This is the most evolutionarily advanced migration of any known butterfly, perhaps of any known insect.
However, climate change and deforestation is threatening their habitats and have greatly eroded the numbers. In response, Mexico has created a 139,000-ace Monarch Butterfly Reserve at a height of 10,000 feet where the butterflies can hibernate for four months. Unfortunately this area is also home to drug cartels that also engage in land theft and the timber trade and recently there were two deaths of reserve workers.
There are plans to plant oyamel firs higher up the mountains so that the temperatures are more suitable and environmentalists hope that the butterflies find them.
Very close to where I live is a smaller monarch butterfly sanctuary where they stop over every October on their migration but I hear that the numbers are far less than what they used to be. My neighbor who have lived here all his life says that it used to be the case that the trees became fully orange because of all the butterflies clustered on them but now it is not so spectacular, though you can still see them.
How animals are able to migrate over vast areas is something that is not fully understood, and it is even more mysterious when they are insects and the migration can span more than one generation.